The invention concerns cementless total hip replacement prostheses (further THR) as replacement of the human hip joint applied by operation.
The term an artificial replacement of the hip joint means mostly a metallic implant and partly a non metallic implant. The replacement of a deformed hip joint by an adequate implant, constructed from special stainless biocompatible steel, cobalt or titanium alloy and nonmetallic materials is the most effective mode of eradicating the deformity or destruction of the hip joint and simultaneously a medicotechnical problem, which has been actual practically since 1923 until now and has not been solved satisfactorily by any prevention (Smith-Petersen, M. V.: Evolution of mold arthroplasty of the hip. J. Bone Joint.Surg.1949, 30B:59-75).
Up to now there have been known several constructions of implants for replacement of deformities or destruction of the hip joint, but each of them have, apart from certain advantages, also disadvantages. The well known THR""s are typically composed of various components: in the acetabulum a metallic implant, inlay of polyethylene or UHMWPE into which interlocks the head of the femoral stem, which has a conical shape implanted with, or without cement in the femur. In dependence from the development in time, various materials have been used for construction of parts of THR. The development has come to stay today on some well defined materials, which have desired utility properties in respect to the hum an organism. These are materials surgical stainless steel, cobalt alloy, titanium alloy, ultra high molecular weight polyethylene, corundum, zirconium ceramics, a layer of metal applied by plasma spray, a metal grid or hydroxylapatite sprayed on the femoral component of the implant, and an inherent part of the material is surgical methylmethacrylate; with or without hydroxylapatite, which is called bone cement in orthopaedics. The mentioned materials have valuable properties with high strength characteristics, low fatigue limits, abrasive resistance and are biocompatible.
The whole entity of known solutions can be divided into 4 basic groups, which can be characterised as follows.
The First Group of THR.
The first group can be characterised as cemented THR (Charnley J. Low friction arthoplasty of the hip. New York: Springer Verlag, 1970). The basic feature of cemented systems is that the THR is composed of the above mentioned parts; the acetabulum and femoral component (stem) being implanted into the place of the original, but non functional hip joint, where they are fixed by bone cement. These two parts are connected to the femoral head manufactured from metal or ceramics and so finishing a new, functional hip joint. From the functional point of view reconstruction of the original abilities of the hip joint occurs and the patient can walk after some time without external support such as canes, crutches etc.
The Second Group of THR.
This group is represented by systems, which could be characterised as cementless THR (Lintner F., Zweymxc3xcller K., Brand G.: Tissue reaction of surrounding to the cementless Ti-6Al-4V after implantation of 7 years. Arch. Orthop Trauma Surg 1988; 1107; 357-366).These are systems with a higher technical and medical effect. Characteristic for this system is its construction from at least two parts, which together create the THR and which is implanted replacing the original-natural, but nonfunctional hip joint. The mentioned components are implanted without cement; the metallic acetabulum is permanently fixed as a xe2x80x9cself-drillingxe2x80x9d screw or by help of typically three or more screws and the femoral stem, optionally sprayed with a biolayer is implanted into the femur. The special biolayer on the femoral stem is created e.g. by plasma spray of metals or hydroxylapatite, or by special techniques from other materials. Such implants belong to the systems, which totally renew the function of the hip joint.
At present it is possible to mention including both groups of THR worldxe2x80x94wide known firms manufacturing and selling THR""s: JOHNSON and JOHNSON (USA), ALLOPRO a Company of SULZER MEDICA (Switzerland), WALDEMAR LINK GmbH and Co (Germany), BIOMET LTD and HOWMEDICA LTD (England), BEZNOSKA s.r.o.(Czech Republic), PROTETIM (Hungary), FEHLING-SEFIDAT (Germany) a.o.
The Third Group of THR.
This group is represented by individual, custom made THR (cementless), which are specially constructed after CT scans of the hip and femur of the patient (Aldinger""s THR by the firm FEHLING, Individual THR HOWMEDICA). These THR""s present the top present state of the art in technics and scientific research in the field of implants; however, the price of these THR""s is very high.
The Fourth Group of THR.
This group is comprised of hybrid THR, containing 2-3 parts, where the acetabular part is mostly fixed by screws or special attachments without cement, or is inserted pressfit and the femoral component is cemented (Fehling-Sefidat).
The THR""s of all four groups can be constructed by unconventional constructing technologies of chip machining or precision casting, or by pressing technologies of metals and plastics using powder technologies and special technologies for creating a spherical head with unevenly of the surface less than 3 to 4 xcexcm.
Several known THR""s have the disadvantage of not being safe against rotation in the transversal plain or against protrusion in vertical direction. Based on scientific research also the application of cement has the disadvantage of possible thermal injury to the living biomaterial during hardening. Often there is a deficiency in securing a homogenous layer of cement around the implant in the femur by classical cementing methods. In connection with preparing the layer of hydroxylapatite there are difficulties in binding of this layer to the metallic surface and in achieving a homogenous thin layer upon the whole functional surface. The greatest disadvantage of the hydroxylapatite layer is the danger of releasing small particles of hydroxylapatite from the surface of the implant, which can lead to abrasion of the polyethylene part or loosening the femoral stem from the femur. It has also been proven that while hydroxylapatite has a good bony ingrowth, by itself it creates an isolation layer with a high electrical and thermal resistance, blocking a fluent passage of biocurrents in that part of the human skeleton, causing subjective and objective difficulties for the bearer of such an implant.
The aim of the present invention is to offer for medical practice such a THR, which eliminates totally or at least in substantial part the mentioned disadvantages.
A cementless total replacement of the human hip joint consists typically of 3 or 4 parts, from which the acetabular component is constructed with a self-drilling thread on its surface and after application is fixed to the pelvis. The femoral component (wedge) with at least one rib and at least one groove is fixed into the femur. Both parts are connected by a head with spherical surface which isn""t subject of the invention and optionally a polyethylene inlay in the acetabulum, which also isn""t subject of the invention.
The substance of the solution resides in the fact that the self-drilling thread is comprised from at least three independent teeth, whereas the outer surface of the mold has advantageously the form of cut spherical or ellipsoid object. The femoral component has in its lower and middle parts a substantially linear inner side and on the outer side or the bottom there is an asymmetrical tapering under the angle xcex3 from the interval of 20 to 70xc2x0 in relation to the axis of the wedge, which ends in a rounded peak of the wedge. In its middle part the femoral component has a trapezoid shape with a wider upper base, which is connected continuously to the upper part containing a convex broadening on the outer side and traverses continuously to the head. Minimally on the lower and middle parts of the inner side of the femoral component there is provided at least one longitudinal groove, the length and depth of which is at least 0.5 mm, and on at least one side surface of the middle and/or upper part of the femoral component there is at least one rib, defining with the axis of the wedge an angle xcex2 in the interval of 70-110xc2x0 the height and width of which is at least 1 mm.
From the viewpoint of implanting the femoral component into the bone it is advantageous if at least on one side surface of the femoral component there is provided at least one longitudinal rib, parallel to the main axis of the wedge.
Advantageous is further, that at least one tooth has a concave shape of the cutting edge, which ensures a good impact into the pelvic bone with only a minimal defect in the bone. The bony particles during cutting are deposited under the sharp edge of the knife and between the teeth and threads of the self-drilling screw. It is also advantageous, if at least one tooth has a blunt edge which relates mainly to the teeth on the beginning of the self-drilling thread, which minimizes the possibility of damaging soft tissues and vessels. From the viewpoint of minimalizing the damage of tissues is even more advantageous, if the edges of the teeth with either sharp or blunt edges are well rounded. It is also advantageous in this connection if the upper edges of the teeth of the self-drilling thread of the mold (acetabulum) are ground in such a manner that their surfaces lie on a spherical or sphere-like surface, corresponding in shape and dimension to the human acetabulum, which greatly diminishes the danger of cutting the teeth through the pelvic bone and so also the danger of later damage of vessels and nerves in the vicinity of the bone.
To ensure an optimal process of the fixation of the mold (acetabulum), the layer of diamond or hydroxylapatite on at least a part of its outer surface is very advantageous. It is also advantageous if at least on the outer part of the femoral component there is a diamond and/or hydroxylapatite layer. In both above mentioned cases is best, if the diamond layer is in the form of a nanocrystalline layer. The creation of a nanocrystalline diamond or hydroxylapatite layer on the outer surface of the acetabulum or femoral component ensures a better ongrowth and ingrowth of bone on the surface, which is a very important property for the bioprocess in the human body. Above all the layer of nanocrystalline diamond is thermally and electrically conductive and has a very good bony ingrowth, whereas from the physical viewpoint it is in good consent with the biomaterial. A further very important advantage of such a layer is its homogeneity within the whole volume. In consequence of its chemical nature, that means it is a form of carbon in the shape of nanocrystalline diamond, which introduces very good properties in connection with the interaction implantxe2x80x94biomaterial. Its thickness can be regulated by a program and is typically in the range of several nanometers, approximately 3-5 nm. An important property of the layer is, also that it makes a strong chemical bond with the metal surface of the wedge.
The substitution of the human hip joint according to the offered solution has, apart from until now described advantages against solutions of the prior art, a fundamental advantage in its preferable biomechanical properties for the patient, higher fatigue and loosening resistances, whereas the femoral component can be constructed with individual dimensions according to the measurement of the upper part of the femur by adequate medical investigations, while maintaining all the described characteristics.